Technical Specification 3GPP TS 24.301 V0.4.0 defines various protocols for communications between User Equipment (UE) and a Mobility Management Entity (MME) within the core network via an E-UTRAN base station (e-NodeB). In general terms, this technical specification defines that whenever the MME receives a Non-Access Stratum (NAS) message “Service Request” from the UE, the MME will always activate Access Stratum (AS) security. The inventors have identified that this may be sub-optimal for some types of services that are provided, such as the Short Messaging Service (SMS).
More specifically, a Service Request procedure is required to trigger the transition from ECM Idle to ECM Connected which establishes the radio and S1 bearers. (ECM stands for EPS (Evolved Packet System) Connection Management.) This procedure is required when:                the network has downlink signalling pending;        the UE has uplink signalling pending;        the UE or the network has user data pending and the UE is in ECM-IDLE mode;        the UE in ECM-CONNECTED mode, has uplink user data to be sent and the user plane radio bearers are not established; or        the UE in ECM-IDLE or ECM-CONNECTED mode and has requested to perform mobile originating/terminating CS fallback.        
FIG. 6 illustrates the message flow between the UE, the eNodeB and the MME for the Short Message Service transfer between the UE and Evolved Packet Core Network. As shown, the UE initially sends an RRC connection Request message to the eNodeB and, once it receives RRC Connection set up message from the e Node B, the UE sends an RRC Connection Setup Complete message together with a NAS: Service Request message. NAS Service Request message may also be sent by the UE in an uplink NAS RRC message. In response to receiving this message, the eNodeB sends the MME an S1AP: Initial UE Message including the NAS message Service Request. This Service Request is a generic request regardless of the service that is ultimately wanted by the UE. In response to receiving the Service Request, the MME activates AS security and transmits an S1AP: Initial UE Context Setup Request back to the eNodeB together with the required Security Parameters and a Service Accept message. This causes the eNodeB to activate security and establish a new Signalling Radio Bearer (SRB2) with the UE. Once this new signalling radio bearer has been established, this new signalling bearer can be used for SMS message transfer between the UE and the network.